Gas sensors are used in many applications, notably in situations where it is desired to detect or recognise a particular gas and in situations where it is desired to determine the composition of a gas mixture. In the present text, unless the context demands otherwise: the expression “gas” will be used to designate both a specific gas species and a mixture of different gaseous species, and the general expression “characterisation” will be used to designate both the process of recognizing or detecting a particular gas and the process of determining the composition of a gas. It is to be understood that references in this text to a “gas sample” generally include references to any gas which is presented to the gas sensor (whether as a discrete sample or by exposing the sensor to an ambient gaseous medium).
Gas sensors have been developed using different sensing technologies, including chemoresistor type gas sensors, such as those based on semi-conducting metal-oxides. FIG. 1 is a cross-sectional view which illustrates, schematically, the basic structure of a semi-conducting metal-oxide type gas sensor 1.
As shown in FIG. 1, a semi-conducting metal-oxide type gas sensor 1 has a sensing layer 2 made of semi-conducting metal-oxide provided on an insulating layer 3 supported on a base 4. When the sensor 1 is exposed to a gas, gas particles G may become adsorbed on the surface of the sensing layer 2, and oxidation-reduction reactions may occur, leading to a change in the impedance (conductance, capacitance, inductance or plural of these parameters) of the sensing layer 2. This change of impedance is measured using a pair of measuring electrodes 5 provided in contact with the sensing layer 2. Often the measurement is made by applying a potential difference across the measurement electrodes and monitoring how the impedance presented by the sensing layer changes. The waveform of the signal produced by the measuring electrodes is characteristic of the gas reacting with the sensing layer 2 and, typically, the waveforms produced by gases of interest are learned during a teaching phase preparatory to analysis of unknown gas samples.
In general, it is necessary to heat the sensing layer 2 to a relatively high temperature (notably 250° C. or above depending on the material forming the sensing layer and the gas species to be detected) for useful adsorption phenomena to be observed. Accordingly, typical gas sensors of this type also include a heater 6, as well as a temperature sensor (not shown in FIG. 1). After a measurement has been taken the heater 6 is activated to heat the active layer to a high temperature, above the usual operating temperature, so as to cause de-sorption of adsorbed particles, thus cleaning the sensor 1 ready for a subsequent measurement.
An aim in this field is to be able to construct micro-sensors, that is to say, miniaturized gas sensors particularly those that are small enough to be integrated into everyday appliances (for example, mobile telephones, face masks, intelligent toys, etc). It is a requirement for micro-sensors that they should have sufficiently high performance, that is, they should be able to detect a target gas, and/or determine a composition of a gas mixture, rapidly and with a sufficiently high degree of accuracy.
Semi-conducting metal-oxide gas sensors attract particular interest for implementation as micro-sensors because they can be built in miniaturized form using techniques known from the field of integrated circuit manufacture.
In recent years semi-conducting metal-oxide type gas sensors having a “micro-hotplate” structure have been developed. FIG. 2(a) is a cross-sectional view which illustrates, schematically, the general structure of a semi-conducting metal-oxide type gas sensor 10 having a micro-hotplate structure. It will be seen from FIG. 2(a) that the base 14 of the sensor 10 has a hollowed-out portion 17 so that the sensing layer 12 is no longer positioned in registration with a thick portion of the base 14. Accordingly, the heater 16 which is used to heat the sensing layer 12 only needs to heat a reduced mass of material (including a relatively thin supporting membrane M), which reduces the power consumed by the gas sensor as well as enabling the temperature of the sensing layer 2 to be increased rapidly (thus reducing the time necessary for making a measurement and reducing the time necessary for cleaning the sensing layer). Moreover, this rapid heating causes less damage to the material forming the sensing layer.
FIGS. 2(b) and 2(c) illustrate sensors having two different types of micro-hotplate architectures.
In the sensor 20 of FIG. 2(b), the sensing layer 22 is formed on an insulating layer 23 which, in its turn, overlies the base 24. Conductors 28 lead out from the measuring electrodes and heater to make contact with electrode pads 29 provided on the base 24. Additional wiring (not shown) connects the electrode pads to further circuitry, notably a source of current for the heater, and circuitry for processing the signals measured by the measurement electrodes. The sensor 20 of FIG. 2(b) has a “closed” type of architecture in which the base 24 has a continuous surface supporting the insulating layer 23.
The sensor 30 illustrated in FIG. 2(c) has a “suspended” type of structure in which the base 34 has a frame-type shape with a central opening 37a and the sensing layer 32 and its insulating layer 33 are suspended over the opening.
Typically, the measurements obtained from a single semi-conducting metal-oxide gas sensor element on its own are insufficient to enable a gas to be identified with a sufficient degree of certainty, because the selectivity of such sensor elements tends to be low. Accordingly, conventionally these sensing elements are used in arrays of multiple sensing elements disposed side-by-side, and each element in the array has a different material forming its sensing layer. The set of measurements obtained from the whole array forms a cloud of data points which can be processed using statistical techniques so as to determine whether or not a given gas is present and/or to determine what is the composition of the gas mixture that has been presented to the array. The set of measurements can be considered to represent a kind of fingerprint that is characteristic of the nature of the gaseous species present in the gas under analysis and their concentrations.
The selectivity and/or the detection-accuracy of an array of semi-conducting metal-oxide gas sensing elements can be improved by increasing the number of data points used in the statistical processing, for example by deriving multiple measurements from each sensing element of the array when it is exposed to a given gas sample. This amounts to obtaining a more detailed fingerprint that is representative of the gas sample. Various techniques are known for obtaining multiple measurements from each sensing element in an array of semi-conducting metal-oxide gas sensing elements and, in general, they involve changing the operating conditions applicable when the various measurements are taken, for example: by measuring the impedance of each sensor element at more than one operating temperature and/or when different profiles of changing temperatures are applied to the sensing layer, by sampling the sensing layer's impedance at different times during its exposure to the gas sample, by measuring the sensing layer's impedance with or without simultaneous exposure to ultraviolet light, etc.
Although micro-sensors using semi-conducting metal-oxide gas-sensing technology and having a micro-hotplate architecture have been developed, addressing demands for small size and speed of measurement, there is a continuing need to improve the accuracy of the results produced by such micro-sensors and, in particular, to improve at least one of their sensitivity and selectivity.
Other micro-sensors have been developed using chemoresistor technology, notably microsensors using conducting polymers. It is desirable to improve the accuracy of the results produced by these sensors also.
More generally, there is a demand to improve the sensitivity and/or selectivity of chemoresistor type gas sensors.